My comment on this is less about the use of the firearm emoji but more along the lines of making sure things look the same no matter what your doing on different devices.
In website design different desktop browsers render the screen differently and on mobile devices you should make sure that the interface still works with the reduced space.
Another thing to look out for if your application runs on different operating systems is fonts being universal. This can change your product from something unique to something terrible.
"The thing is, emojis already look different on different platforms and it does cause confusion," Jeremy Burge, editor of Emojipedia, told the BBC. "When we're dealing with guns and toys as a comparison, that's a whole new level of problems that we have there." In his blog, Mr Burge pointed out that someone could send a message from an iOS device mentioning a water pistol - but that this could look like they were asking the recipient to bring a real weapon if that user did not have iOS.